1. Field of the Invention
The present invention relates to a structure of a photomask used in an exposure process. More particularly, the present invention relates to a structure of a phase shift photomask that is capable of reducing the line-end shortening (LES) effect.
2. Description of the Related Art
As the linewidth of semiconductor process rapidly goes down, shortening the wavelength of exposure light in lithography is always desired. However, since short-wavelength exposure systems are difficult to design, most of advanced processes have linewidths shorter than the wavelength of the exposure light. For example, in an advanced 90 nm process, ArF excimer laser of 193 nm is usually used as an exposure light source.
However, when the process linewidth goes down to approximately one half of the exposure wavelength, the resolution of an exposure process using a conventional binary photomask is insufficient. Therefore, various phase shift photomasks are developed, which utilize optical interference effect to enhance the contrast of pattern transfer and improve the resolution of exposure process.
For example, a half-tone (HT) phase shift photomask as shown in FIG. 1 is usually used to form isolated and dense line patterns. The HT photomask includes a transparent substrate 100 and isolated/dense line patterns 110/120, which are semi-transparent lines with a phase shift of 180° relative to the substrate 100. Nevertheless, significant line-end shortening (LES) effect still occurs to the isolated photoresist line pattern 12 and the dense photoresist line patterns 14 obtained by using the above HT photomask, as indicated by LESI and LESD in FIG. 1. Consequently, electrically connecting with the conductive lines defined by the isolated/dense photoresist line patterns is difficult.